1. Field of the Invention
This invention pertains to drying wet carbonaceous material such as coal and/or lignite so as to passivate it and prevent significant reabsorption of atmospheric moisture and consequent reheating of the material. It pertains more particularly to drying such materials at slow controlled rate below a critical moisture level, so that internal moisture is removed slowly to avoid particle rupture and the particle surface pores are substantially self-sealed by hydrocarbon material evolved from the particles.
2. Description of Prior Art
Low rank coals such as sub-bituminous or lignite contain more than about 10% moisture and usually 15-50 weight percent moisture. Such wet coals cannot be shipped economically over great distances due to the large water weight penalty and/or be burned efficiently due to the significant negative heating value of the water. However, drying such materials prior to shipment and/or storage usually leads to substantial reabsorption of atmospheric moisture and consequential reheating of the coal, which makes it subject to spontaneous ignition either during shipment or subsequent storage.
In the past, wet coals such as from western United States and lignite have been dried and then coated with carbonaceous materials such as heavy oil and tars to retard or prevent the reabsorption of moisture and possible reheating and autoignition. Examples of such coating processes are U.S. Pat. Nos. 3,985,516 and 3,985,517 to Johnson, which disclose heating and intimate mixing of coal with heavy oils to coat the particles. Although such coating procedures are rather effective in preventing reabsorption of moisture by the dried coal, such oil coating of dried coal and lignite is expensive due to the cost of the hydrocarbon materials added and thus is unattractive. It would be advantageous to dry wet coals in such a way that the coal particle pores are made self-sealing after moisture removal, so as to prevent the reabsorption of moisture without the need for externally supplied coating materials.
U.S. Pat. No. 1,632,829 to Fleissner discloses a process for drying wet coal by steam heating it using a procedure wherein steam provided above the coal is maintained at high partial pressure such that moisture will not escape during coal heat up, then reducing the steam pressure to permit the escape of moisture and rapid drying of the coal. Also, U.S. Pat. No. 4,052,169 to Koppelman discloses a process for upgrading lignitic coal, comprising heating it in an autoclave at about 750.degree. F. temperature and 1000 psig or more pressure to effect a thermal restructuring, followed by cooling and depositing condensible organic material on the lignite to provide a stabilization of the upgraded product and render it nonhydroscopic and more resistant to weathering and oxidation during shipment and storage. In contrast, the present invention achieves rapid removal of surface moisture from the coal during its initial low pressure heating, and later permits the inner bound moisture to escape at a controlled slow rate so as to prevent surface rupture of the coal, thus permitting self-sealing of the particle surface by internal hydrocarbon material which is wicked up to the surface.